My son's deer gun is a Marlin 336 chambered in .35 Remington,
the receiver is not drilled or tapped for a scope. I want to mount
a red dot on it for him; where we hunt (WI) most shots are under
100 yds and often under 50 yds. Heavy woods. Last year during
the early season (anlterless) we had a fork buck at 20 feet.

I have a machine shop out back and may do the work myself. Any
advise on how to get this done would be appreciated.

It would be great if you had a 336 drilled & tapped so you could measure the hole spacing. Find exact center of the receiver (from side to side). Make SURE the receiver is in the padded vise exactly vertical. Mark your holes. Set drill stop. Drill hole (#6 drill bit). Attach tap to your drill press and tap hole using a regular tap (starter) and bottoming tap. The size should be 6x48.

It would be great if you had a 336 drilled & tapped so you could measure the hole spacing. Find exact center of the receiver (from side to side). Make SURE the receiver is in the padded vise exactly vertical. Mark your holes. Set drill stop. Drill hole (#6 drill bit). Attach tap to your drill press and tap hole using a regular tap (starter) and bottoming tap. The size should be 6x48.

Thanks, do you know the hole depth?

__________________An Armed man is a Citizen,
An Unarmed man is a Subject. -- Unknown

Shade,
The scope mount holes on the Marlin 336 are drilled and tapped for 8-40 screws. All of the holes are drilled and tapped prior to the barrel being installed and are all through holes. When drilling a barreled reciever, the front hole is drilled to a depth of .130", the other three are through holes.
Below is a drawing of the hole spacings from a factory drilled reciever.